I'm a little confused as to what are the difference between them, when should I use which?

Also, how long is the lifespan and proper care after opening of these products? If I don't bake that often can I just keep a small can in the pantry for a long while? Can they be substituted for each other?

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Most of these questions are individually answered, or at least worth exploring. I have to vote to close your question as to broad, but I invite you explore the many questions in the yeast and baking-powder, as well as some of the more pertinent ones linked below. If you still have a question that is more specific, please do ask it.
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SAJ14SAJOct 19 '13 at 18:57

I agree with SAJ14SAJ, these questions together make a sizeable book chapter, if not a small book on food technology. So, sorry, we have to close it as too broad.
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rumtscho♦Oct 19 '13 at 19:07